Nonvolatile Memory Device and Read Methods Thereof

ABSTRACT

An object of the present inventive concept is providing a nonvolatile memory device having improved reliability by compensating a threshold voltage of a flash memory cell. 
     A nonvolatile memory device according to the present inventive concept includes a memory cell array connected to a plurality of word lines; and a voltage generator for supplying a select read voltage to a select word line and an unselect read voltage to an unselect word line among the plurality of word lines when a read operation is performed. The voltage generator generates the unselect read voltage having a different level according to whether the unselect word line is adjacent to the select word line or not. 
     A nonvolatile memory device according to the present inventive concept compensates a threshold voltage increased or decreased due to various causes. According to the present inventive concept, reliability of a nonvolatile memory device is improved.

REFERENCE TO PRIORITY APPLICATION

This patent application claims priority to Korean Patent Application No.2008-0113531, filed Nov. 14, 2008, the contents of which are herebyincorporated herein by reference.

FIELD OF THE INVENTION

The exemplary embodiments disclosed herein relate to nonvolatile memorydevices, and more particularly, to nonvolatile memory devices includingimproved reliability and methods of reading the same.

BACKGROUND

Memory devices are used to store data. The memory devices are classifiedinto volatile memory devices and nonvolatile memory devices. Data storedin volatile memory devices are disappeared when their power supplies areinterrupted while data stored in nonvolatile memory devices are retainedeven when their power supplies are interrupted.

Since nonvolatile memory devices can store data using a low power, theyare in the limelight as a storage medium of a portable device. There isa flash memory device as a kind of a nonvolatile memory device. Inbelow, a flash memory device is described as an example of a nonvolatilememory device. However, a range of the present inventive concept is notlimited to this but can be applied to other nonvolatile memory devices(e.g., a charge trap flash memory, PRAM, FRAM, MRAM or the like).

Data stored in a memory cell of a flash memory device (or a charge trapflash memory) is distinguished by measuring a threshold voltage of amemory cell. A threshold voltage of a memory cell is determineddepending on the number of electrons stored in a floating gate. As thenumber of electrons stored in a floating gate increases, a thresholdvoltage becomes high.

Electrons stored in a floating gate may be leaked by many causes.Electrons stored in a floating gate may be leaked by an externalstimulus (e.g., heat). Also, electrons stored in a floating gate may beleaked by wear of a memory cell. A repetition of an access operation ina flash memory device wears an insulating layer disposed between achannel region and a floating gate. The access operation includes aprogram operation, an erasing operation and a read operation. If aninsulating layer is worn, charges stored in a floating gate are easilyleaked. A decrease of a threshold voltage causes a read error, therebydegrading reliability of a flash memory device.

SUMMARY

Exemplary embodiments of the present inventive concept provide anonvolatile memory device. The nonvolatile memory device may include amemory cell array connected to a plurality of word lines; and a voltagegenerator for supplying a select read voltage to a select word line andan unselect read voltage to an unselect word line among the plurality ofword lines when a read operation is performed. The voltage generatorgenerates the unselect read voltage having a different level accordingto whether the unselect word line is adjacent to the select word line ornot.

Exemplary embodiments of the present inventive concept provide a readmethod of a nonvolatile memory device. The read method may includeperforming a first read operation by supplying a select read voltage toa select word line and supplying an unselect read voltage to an unselectword line; detecting whether an error occurs in the first read operationor not; and performing a second read operation by changing a voltage ofa word line adjacent to the select word line when an error occurs in thefirst read operation.

Exemplary embodiments of the present inventive concept provide a readmethod of a nonvolatile memory device. The read method may includeperforming a read operation by supplying an select read voltage to amonitoring word line and supplying an unselect read voltage to anunselect word line; detecting whether data stored in a monitoring cellconnected to the monitoring word line is normally readout or not; andperforming a read operation on the monitoring cell again by changing avoltage of a word line adjacent to the monitoring word line when datastored in the monitoring cell is not normally readout.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures are included to provide a further understandingof the present invention, and are incorporated in and constitute a partof this specification. The drawings illustrate exemplary embodiments ofthe present invention and, together with the description, serve toexplain principles of the present invention. In the figures:

FIG. 1 is a cross section view illustrating a memory cell of a flashmemory device.

FIG. 2A and FIG. 2B are cross section views illustrating a charge trapflash memory device respectively.

FIG. 3 is a graph illustrating a distribution of a threshold voltage ofa memory cell.

FIG. 4 is a graph illustrating a case that the threshold voltage of thememory cell depicted in FIG. 3 is decreased.

FIG. 5 is a graph illustrating a distribution of a threshold voltage ofa three bit multi level cell (MLC).

FIG. 6 is a graph illustrating a case that the threshold voltage of thethree bit multi level cell (MLC) depicted in FIG. 5 is decreased.

FIG. 7 is a block diagram illustrating a nonvolatile memory deviceaccording to the present inventive concept.

FIG. 8 is a drawing illustrating an effect which a select memory cellreceives from an upper word line and a lower word line adjacent to theselect memory cell.

FIG. 9 is a graph illustrating a change of a threshold voltage of aselect memory cell according to a read voltage applied to an upper wordline and a lower word line when a read operation is performed.

FIG. 10 is a drawing illustrating a bias condition of when verificationread operation of a nonvolatile memory device according to the presentinventive concept is performed.

FIG. 11 is a drawing illustrating a bias condition of when a readoperation according to a first embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed.

FIG. 12 is a drawing illustrating a bias condition of when a readoperation according to a second embodiment of a nonvolatile memorydevice according to the present inventive concept is performed.

FIG. 13 is a drawing illustrating a bias condition of when a readoperation according to a third embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed.

FIG. 14 is a graph illustrating a change of a threshold voltage of amemory cell according to the bias conditions depicted in FIGS. 11 to 13.

FIG. 15 is a drawing illustrating a bias condition of when a readoperation according to a fourth embodiment of a nonvolatile memorydevice according to the present inventive concept is performed.

FIG. 16 is a drawing illustrating a bias condition of when a readoperation according to a fifth embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed.

FIG. 17 is a drawing illustrating a bias condition of when a readoperation according to a sixth embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed.

FIG. 18 is a graph illustrating a change of a threshold voltage of amemory cell according to the bias conditions depicted in FIGS. 15 to 17.

FIG. 19 is a flow chart illustrating a read method according to aneighth embodiment of a nonvolatile memory device according to thepresent inventive concept.

FIG. 20 is a flow chart illustrating a read method according to a ninthembodiment of a nonvolatile memory device according to the presentinventive concept.

FIG. 21 is a flow chart illustrating a read method according to a tenthembodiment of a nonvolatile memory device according to the presentinventive concept.

FIG. 22 is a flow chart illustrating a read method according to aeleventh embodiment of a nonvolatile memory device according to thepresent inventive concept.

FIG. 23 is a flow chart illustrating a read method according to atwelfth embodiment of a nonvolatile memory device according to thepresent inventive concept.

FIG. 24 is a flow chart illustrating a read method according to athirteenth embodiment of a nonvolatile memory device according to thepresent inventive concept.

FIG. 25 is a block diagram illustrating a computing system including aflash memory according to the present inventive concept.

FIG. 26 is a block diagram illustrating a structure of SSD systemaccording to the present inventive concept.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstructed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the present inventionto those skilled in the art. Like numbers refer to like elementsthroughout.

FIG. 1 is a cross section view illustrating a memory cell of a flashmemory device. Referring to FIG. 1, a source (S) and a drain (D) areformed in a semiconductor substrate of both sides of a channel region. Afloating gate is formed on the channel region and an insulating layer isdisposed between the channel region and the floating gate. A controlgate is formed on the floating gate and an insulating layer is disposedbetween the floating gate and the control gate. Terminals for applyingvoltages for a program operation, an erasing operation and a readoperation are connected to the source (S), the drain (D), the floatinggate, the control gate and the semiconductor substrate.

FIG. 2A and FIG. 2B are cross section views illustrating a charge trapflash memory device. Referring to FIG. 2A, a tunnel oxide layer 130, acharge trap layer 140, a blocking insulating layer 150 and a gateelectrode 160 are sequentially formed on a substrate 110 including asource/drain region 120. The charge trap layer 140 is formed of nitridesilicon, the blocking insulating layer 150 is formed of oxide aluminumand the gate electrode 160 is formed of nitride tantalum. A charge trapflash memory device having this structure is a kind of MONOS, especiallyis referred to as Thallium-Aluminum oxide-Nitride-Oxide-Semiconductor(TANOS).

Referring to FIG. 2B, a tunnel oxide layer 130 a, a charge trap layer140 a, a blocking insulating layer 150 a and a gate electrode 160 a aresequentially formed on a substrate 110 a including a source/drain region120 a. In this case, the charge trap layer 140 a is formed of laminatedthree layers. That is, a first nitride silicon layer 142 a, a firstoxide aluminum layer 144 a and a second nitride silicon layer 46 a aresequentially stacked. The blocking insulating layer 150 a is formed ofoxide aluminum and the gate electrode 160 a is formed of nitridetantalum. A charge trap flash memory device having this structure isreferred to as Nitride-Aluminumoxide-Nitride (NAN).

In the flash memory device described above, data is decoded bydistinction of a threshold voltage of a memory cell. A threshold voltageof a memory cell is determined by the amount of electrons stored in afloating gate or a charge trap layer. As the number of electrons storedin a floating gate or a charge trap layer increases, a threshold voltagebecomes high.

Electrons stored in a floating gate may be leaked in the direction ofthe arrow of FIG. 1 by many causes. Electrons stored in a floating gateor a charge trap layer may be leaked by an external stimulus (e.g.,heat). Also, electrons stored in a floating gate or a charge trap layermay be leaked by wear of a memory cell. A repetition of an accessoperation in a flash memory device wears an insulating layer between achannel region and a floating gate. The access operation includes aprogram operation, an erasing operation and a read operation. If aninsulating layer is worn, charges stored in a floating gate are easilyleaked.

FIG. 3 is a graph illustrating a distribution of a threshold voltage ofa memory cell. Referring to FIG. 3, a horizontal axis represents athreshold voltage (Vth) and a vertical axis represents the number ofmemory cells. In a case of a single level cell (SLC), a thresholdvoltage of a memory cell has one between two states (‘S0’, ‘S1’).

When a read voltage (Vr) is applied to a control gate (referring toFIG. 1) of a memory cell, a memory cell of ‘S0’ state is turned on. Incontrast, a memory cell of ‘S1’ state is turned off. When a memory cellis turned on, a current flows through the memory cell. When a memorycell is turned off, a current does not flow through the memory cell.Thus, data can be distinguished depending on whether a memory cell isturned on or not. A threshold voltage of a memory cell should beuniformly maintained so as to accurately detect data stored in a memorycell. However, as described above, a threshold voltage of a memory cellmay be decreased by an external environment and/or wear of a memorycell.

FIG. 4 is a graph illustrating a case that the threshold voltage of thememory cell depicted in FIG. 3 is decreased. Referring to FIG. 4, asolid line represents an initial threshold voltage of a memory cell anda dotted line represents a threshold voltage of a memory cell decreasedby an external environment and/or wear of a memory cell. Memory cellswhich belong to a diagonal portion of FIG. 4 are originally programmedin ‘Si’ state but they may be judged to be programmed in ‘S0’ state by adecrease of a threshold voltage. This may cause a read error, therebydegrading reliability of a nonvolatile memory device. A change of athreshold voltage causes problems, especially in a multi level cell(MLC). A plurality of data bits is stored in a multi level cell (MLC) soas to increase an integration of a nonvolatile memory device.

FIG. 5 is a graph illustrating a distribution of a threshold voltage ofa three bit multi level cell (MLC). Referring to FIG. 5, a thresholdvoltage of the three bit multi level cell (MLC) is one among eightstates (‘S0’˜‘S7’). ‘S0’ represents an erasure state and ‘S1’∞‘S7’represent a program state. A threshold voltage margin of a multi levelcell (MLC) is small as compared with a single level cell (SLC).Accordingly, a small change of a threshold voltage in a multi level cell(MLC) may cause a big problem.

FIG. 6 is a graph illustrating a case that the threshold voltage of thethree bit multi level cell (MLC) depicted in FIG. 5 is decreased.Referring to FIG. 6, a solid line represents an initial thresholdvoltage of a memory cell and a dotted line represents a thresholdvoltage of a memory cell decreased by an external environment and/orwear of a memory cell. A read error may occur by a decrease of athreshold voltage when reading memory cells corresponding to a diagonalportion.

FIG. 7 is a block diagram illustrating a nonvolatile memory deviceaccording to the present inventive concept. Referring to FIG. 7, anonvolatile memory device includes a memory cell array 210, a row selectcircuit 220, an input/output circuit 250, a voltage generator 230 and acontrol logic circuit 240.

The cell array includes a plurality of memory blocks (not shown). In aflash memory, an erasure operation may be performed on each block. Eachblock is comprised of memory cells arranged in the form of a matrix ofrow (or word lines) and columns (or bit lines). Memory cells may bearranged to have a NAND structure or a NOR structure.

The row select circuit 220 drives a selected row and unselected rowsrespectively in response to a row address (not shown). A drive voltageis generated by the voltage generator 230. When a read operation isperformed, the row select circuit 220 applies a read reference voltageto a selected row and a read voltage (Vread) to unselected rows.

The input/output circuit 250 operates as a sense amplifier when a readoperation is performed. When a read operation is performed, theinput/output circuit 250 readouts data from the memory cell. The controllogic circuit 240 controls the voltage generator 230 and theinput/output circuit 250 to readout data stored in the memory cell 210.

FIG. 8 is a drawing illustrating an effect which a select memory cellreceives from an upper word line and a lower word line adjacent to theselect memory cell. A select memory cell means a memory cell whichbecomes a target of a read operation. A select word line means a wordline to which a select memory cell is connected. In an embodimentaccording to the present inventive concept, a read voltage applied to anupper word line (WLn+1) and a lower word line (WLn−1) of a select wordline (WLn) is controlled by a change of a threshold voltage of a selectmemory cell.

A channel voltage of a select memory cell connected to a select wordline (WLn) may be affected by a voltage of the upper word line (WLn+1)and a voltage of the lower word line (WLn−1). As an integration of anonvolatile memory device is improved, a degree of interference by avoltage of an adjacent word line becomes great. Also, a channel voltageof a select memory cell is affected not only by a select word linevoltage (WLn) but also by an adjacent upper word line voltage (WLn+1)and an adjacent lower word line voltage (WLn−1). This can change athreshold voltage of a select memory cell. For example, when a readoperation of a nonvolatile memory device is performed, a select memorycell may be affected by a read voltage (Vread) applied to the upper wordline (WLn+1) and the lower word line (WLn−1).

FIG. 9 is a graph illustrating a change of a threshold voltage of aselect memory cell according to a read voltage applied to an upper wordline and a lower word line when a read operation is performed. Referringto FIG. 9, Vread means a voltage applied to an unselect word line whenprogram verification operation is performed. Memory cells connected toan unselect word line are turned on by a Vread. If a current Iref flowsthrough a memory cell, the memory cell is recognized to be turned on anda gate voltage at this time becomes a threshold voltage.

In the present embodiment, when a read operation is performed, Vreadvoltage is applied to an upper word line and a lower word line of aselect word line. A case of (1) shows a case of applying Vread′ higherthan Vread to an upper word line and a lower word line of a select wordline when a read operation is performed. At this time, the current Irefflows through a select memory cell when a gate voltage of a selectmemory cell is Vth1. That is, Vth1 becomes a threshold voltage.

A case of (2) shows a case of applying Vread′ equal to Vread to an upperword line and a lower word line of a select word line when a readoperation is performed. At this time, the current Iref flows through aselect memory cell when a gate voltage of a select memory cell is Vth2.That is, Vth2 becomes a threshold voltage.

A case of (3) shows a case of applying Vread' lower than Vread to anupper word line and a lower word line of a select word line when a readoperation is performed. At this time, the current Iref flows through aselect memory cell when a gate voltage of a select memory cell is Vth3.That is, Vth3 becomes a threshold voltage.

In summary, a threshold voltage of a select memory cell is changeddepending on a read voltage (Vread′) applied to an upper word line and alower word line of a select word line when a read operation isperformed. Thus, a threshold voltage of a select memory cell can becompensated by controlling a read voltage (Vread′) applied to an upperword line and a lower word line of a select word line when a readoperation is performed.

FIG. 10 is a drawing illustrating a bias condition of when verificationread operation of a nonvolatile memory device according to the presentinventive concept is performed. Verification read operation is performedso as to detect whether a memory cell is normally programmed or not.Referring to FIG. 10, for convenience, one memory cell string isdepicted. A memory cell array includes a plurality of cell strings. Amemory cell string includes a string select transistor connected to astring select line (SSL), a floating gate transistor connected to eachof word lines (WL0˜WL31) and a ground select transistor connected to aground select line (GSL).

When verification read operation is performed, a power supply voltage(Vcc) is applied to a string select line (SSL). As a power supplyvoltage (Vcc) is applied, a string select transistor is turned on. Thus,a memory cell string is connected to a bit line (BL). A read voltage(Vread) is applied to unselect word lines. As a read voltage (Vread) isapplied, unselect memory cells connected to unselect word lines areturned on.

A verification voltage (Vvf) is applied to a select word line. As averification voltage (Vvf) is applied, a select memory cell is turned onor turned off according to a data state. A select memory cell which isprogrammed is turned off and a select memory cell which is notprogrammed is turned on. A power supply voltage (Vcc) is applied to aground select line (GSL). As a power supply voltage (Vcc) is applied, aground select transistor is turned on. Thus, a memory cell string isconnected to a common source line (CSL).

FIG. 11 is a drawing illustrating a bias condition of when a readoperation according to a first embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed. Referring toFIG. 11, when a read operation is performed, a power supply voltage(Vcc) is applied to a string select line (SSL). As a power supplyvoltage (Vcc) is applied, a string select transistor is turned on. Thus,a memory cell string is connected to a bit line (BL). A read voltage(Vread) is applied to unselect word lines. As a read voltage (Vread) isapplied, unselect memory cells connected to unselect word lines areturned on.

A read reference voltage (Rx) is applied to a select word line (WL28).As a read reference voltage (Rx) is applied, a select memory cell isturned on or turned off according to a data state. A select memory cellwhich is programmed is turned off and a select memory cell which is notprogrammed is turned on. A power supply voltage (Vcc) is applied to aground select line (GSL). As a power supply voltage (Vcc) is applied, aground select transistor is turned on. Thus, a memory cell string isconnected to a common source line (CSL).

In an embodiment according to the present inventive concept, Vread′lower than Vread is applied to an upper word line (WL29) and a lowerword line (WL27) of the select word line (WL28). A threshold voltage ofa select memory cell connected to the select word line (WL28) isincreased by interferences of voltages of the upper word line (WL29) andthe lower word line (WL27). Thus, a threshold voltage of a select memorycell lowered by a current leakage can be compensated.

FIG. 12 is a drawing illustrating a bias condition of when a readoperation according to a second embodiment of a nonvolatile memorydevice according to the present inventive concept is performed.Referring to FIG. 12, when a read operation is performed, a power supplyvoltage (Vcc) is applied to a string select line (SSL). As a powersupply voltage (Vcc) is applied, a string select transistor is turnedon. Thus, a memory cell string is connected to a bit line (BL). A readvoltage (Vread) is applied to unselect word lines. As a read voltage(Vread) is applied, unselect memory cells connected to unselect wordlines are turned on.

A read reference voltage (Rx) is applied to a select word line (WL28).As a read reference voltage (Rx) is applied, a select memory cell isturned on or turned off according to a data state. A select memory cellwhich is programmed is turned off and a select memory cell which is notprogrammed is turned on. A power supply voltage (Vcc) is applied to aground select line (GSL). As a power supply voltage (Vcc) is applied, aground select transistor is turned on. Thus, a memory cell string isconnected to a common source line (CSL).

In an embodiment according to the present inventive concept, Vread′lower than Vread is applied to a lower word line (WL27) of the selectword line (WL28). A threshold voltage of a select memory cell connectedto the select word line (WL28) is increased by an interference of avoltage of the lower word line (WL27). Thus, a threshold voltage of aselect memory cell lowered by a current leakage can be compensated.

FIG. 13 is a drawing illustrating a bias condition of when a readoperation according to a third embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed. Referring toFIG. 13, when a read operation is performed, a power supply voltage(Vcc) is applied to a string select line (SSL). As a power supplyvoltage (Vcc) is applied, a string select transistor is turned on. Thus,a memory cell string is connected to a bit line (BL). A read voltage(Vread) is applied to unselect word lines. As a read voltage (Vread) isapplied, unselect memory cells connected to unselect word lines areturned on.

A read reference voltage (Rx) is applied to a select word line (WL28).As a read reference voltage (Rx) is applied, a select memory cell isturned on or turned off according to a data state. A select memory cellwhich is programmed is turned off and a select memory cell which is notprogrammed is turned on. A power supply voltage (Vcc) is applied to aground select line (GSL). As a power supply voltage (Vcc) is applied, aground select transistor is turned on. Thus, a memory cell string isconnected to a common source line (CSL).

In an embodiment according to the present inventive concept, Vread′lower than Vread is applied to an upper word line (WL29) of the selectword line (WL28). A threshold voltage of a select memory cell connectedto the select word line (WL28) is increased by an interference of avoltage of the upper word line (WL29). Thus, a threshold voltage of aselect memory cell lowered by a current leakage can be compensated.

FIG. 14 is a graph illustrating a change of a threshold voltage of amemory cell according to the bias conditions depicted in FIGS. 11 to 13.Referring to FIG. 14, a dotted line shows a decrease of a thresholdvoltage of a memory cell by a leakage current. In an embodimentaccording to the present inventive concept, a threshold voltage of amemory cell is lowered by applying Vread′ lower than a read voltage(Vread) to an upper word line and/or a lower word line of a select wordline. Thus, a threshold voltage of a select memory cell lowered by acurrent leakage can be compensated.

In a case of a conventional nonvolatile memory device, a decreasedthreshold voltage of a memory cell can be compensated by changing a readreference voltage. In contrast, in the present inventive concept, a readoperation can be performed without a change of a read reference voltageby increasing a threshold voltage of a memory cell.

However, a threshold voltage of a memory cell can be increased as thecase may be. For example, a threshold voltage of a memory cell can beincreased by a coupling due to an adjacent memory cell. An increasedthreshold voltage of a memory cell may reduce an accuracy of a readoperation and may degrade reliability of a nonvolatile memory device.Hereinafter, a read method of when a threshold voltage of a memory cellaccording to the present inventive concept is increased will bedescribed in detail referring to reference drawings.

FIG. 15 is a drawing illustrating a bias condition of when a readoperation according to a fourth embodiment of a nonvolatile memorydevice according to the present inventive concept is performed.Referring to FIG. 15, when a read operation is performed, a power supplyvoltage (Vcc) is applied to a string select line (SSL). As a powersupply voltage (Vcc) is applied, a string select transistor is turnedon. Thus, a memory cell string is connected to a bit line (BL). A readvoltage (Vread) is applied to unselect word lines. As a read voltage(Vread) is applied, unselect memory cells connected to unselect wordlines are turned on.

A read reference voltage (Rx) is applied to a select word line (WL28).As a read reference voltage (Rx) is applied, a select memory cell isturned on or turned off according to a data state. A select memory cellwhich is programmed is turned off and a select memory cell which is notprogrammed is turned on. A power supply voltage (Vcc) is applied to aground select line (GSL). As a power supply voltage (Vcc) is applied, aground select transistor is turned on. Thus, a memory cell string isconnected to a common source line (CSL).

In an embodiment according to the present inventive concept, Vread′higher than Vread is applied to an upper word line (WL29) and a lowerword line (WL27) of the select word line (WL28). A threshold voltage ofa select memory cell connected to the select word line (WL28) islowered. Thus, a threshold voltage of a select memory cell increased bya coupling can be compensated.

FIG. 16 is a drawing illustrating a bias condition of when a readoperation according to a fifth embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed. Referring toFIG. 16, when a read operation is performed, a power supply voltage(Vcc) is applied to a string select line (SSL). As a power supplyvoltage (Vcc) is applied, a string select transistor is turned on. Thus,a memory cell string is connected to a bit line (BL). A read voltage(Vread) is applied to unselect word lines. As a read voltage (Vread) isapplied, unselect memory cells connected to unselect word lines areturned on.

A read reference voltage (Rx) is applied to a select word line (WL28).As a read reference voltage (Rx) is applied, a select memory cell isturned on or turned off according to a data state. A select memory cellwhich is programmed is turned off and a select memory cell which is notprogrammed is turned on. A power supply voltage (Vcc) is applied to aground select line (GSL). As a power supply voltage (Vcc) is applied, aground select transistor is turned on. Thus, a memory cell string isconnected to a common source line (CSL).

In an embodiment according to the present inventive concept, Vread′higher than Vread is applied to a lower word line (WL27) of the selectword line (WL28). A threshold voltage of a select memory cell connectedto the select word line (WL28) is lowered. Thus, a threshold voltage ofa select memory cell increased by a coupling can be compensated.

FIG. 17 is a drawing illustrating a bias condition of when a readoperation according to a sixth embodiment of a nonvolatile memory deviceaccording to the present inventive concept is performed. Referring toFIG. 17, when a read operation is performed, a power supply voltage(Vcc) is applied to a string select line (SSL). As a power supplyvoltage (Vcc) is applied, a string select transistor is turned on. Thus,a memory cell string is connected to a bit line (BL). A read voltage(Vread) is applied to unselect word lines. As a read voltage (Vread) isapplied, unselect memory cells connected to unselect word lines areturned on.

A read reference voltage (Rx) is applied to a select word line (WL28).As a read reference voltage (Rx) is applied, a select memory cell isturned on or turned off according to a data state. A select memory cellwhich is programmed is turned off and a select memory cell which is notprogrammed is turned on. A power supply voltage (Vcc) is applied to aground select line (GSL). As a power supply voltage (Vcc) is applied, aground select transistor is turned on. Thus, a memory cell string isconnected to a common source line (CSL).

In an embodiment according to the present inventive concept, Vread′higher than Vread is applied to an upper word line (WL29) of the selectword line (WL28). A threshold voltage of a select memory cell connectedto the select word line (WL28) is lowered. Thus, a threshold voltage ofa select memory cell increased by a coupling can be compensated.

FIG. 18 is a graph illustrating a change of a threshold voltage of amemory cell according to the bias conditions depicted in FIGS. 15 to 17.Referring to FIG. 18, a dotted line shows an increase of a thresholdvoltage of a memory cell by a coupling. In an embodiment according tothe present inventive concept, a threshold voltage of a memory cell islowered by applying Vread′ higher than a read voltage (Vread) to anupper word line and/or a lower word line of a select word line. Thus, athreshold voltage of a select memory cell increased by a coupling can becompensated.

In a case of a conventional nonvolatile memory device, an increasedthreshold voltage of a memory cell can be compensated by changing a readreference voltage. In contrast, in the present inventive concept, a readoperation can be performed without a change of a read reference voltageby decreasing a threshold voltage of a memory cell. This simplifies astructure of a nonvolatile memory device.

However, in a flash memory device, a threshold voltage of a memory cellmay decrease immediately after a program pass on a memory cell. Thisphenomenon is caused by a phenomenon that after a charge is trapped in amoment by a program of a memory cell, a charge is detrapped in a momentagain. Thus, when a read operation is performed immediately afterfinishing a program and a program verification operation of a memorycell, a problem that a memory cell may be judged not to be programmedmay occur.

The problem described above may be removed by a seventh embodimentaccording to the present inventive concept. When a read operation isperformed, Vread′ lower than Vread that is applied when a programverification operation is performed is applied to an upper word line anda lower word line. The amount of an initial decrease of a thresholdvoltage can be obtained as a type of a statistical data. Thus, referringto a statistical data, the amount of an initial decrease of a thresholdvoltage according to the number of a program repetition is determined inadvanced and Vread′ according to the amount of an initial decrease maybe applied. A statistical data may be stored inside or outside of anonvolatile memory device.

FIG. 19 is a flow chart illustrating a read method according to aneighth embodiment of a nonvolatile memory device according to thepresent inventive concept. Referring to FIG. 19, a read method of anonvolatile memory device according to the present inventive conceptincludes a step of a read operation performance, an error detection stepand a step of a changing a read voltage.

In S110 step, a nonvolatile memory device receives a read instructionfrom external device. For example, a nonvolatile memory device mayreceive a read instruction from a memory controller. In S120 step, anonvolatile memory device performs a read operation in response to aread instruction. A read operation is performed using a well known biascondition. In S130 step, an error occurs during a read operation and ifan error occurred can not be corrected, S140 step is performed. Incontrast, if an error does not occur or an error occurred can becorrected, a read operation is over.

In S140 step, a read voltage (Vread′) applied to an upper word line anda lower word line of a select word line is changed. For example, when athreshold voltage of a memory cell decreases, a read voltage (Vread′) isdecreased and when a threshold voltage of a memory cell increases, aread voltage (Vread′) is increased. A changed read voltage (Vread′) isstored in a nonvolatile memory controller, a volatile memory controllerand a flash memory controller and can be used during a next readoperation. In S150 step, a read operation is performed again by applyinga changed read voltage (Vread′) to an upper word line and a lower wordline. In S160 step, an error occurs during a read operation and if anerror occurred can not be corrected, S140 step is performed again. Incontrast, if an error does not occur or an error occurred can becorrected, a read operation is over.

In an embodiment according to the present inventive concept, a level ofa read voltage (Vread′) is changed according to whether an erroroccurred during a read operation can be corrected or not. A reduction ofa memory cell or an increased threshold voltage can be compensated bychanging a read voltage (Vread′).

FIG. 20 is a flow chart illustrating a read method according to a ninthembodiment of a nonvolatile memory device according to the presentinventive concept. Referring to FIG. 20, a read method of a nonvolatilememory device according to the present inventive concept includes a stepof a read operation performance, an error detection step, a step of achanging a read voltage and a step of changing a read reference voltage.

In S205 step, a nonvolatile memory device receives a read instructionfrom external device. In S210 step, a nonvolatile memory device performsa read operation in response to a read instruction. A read operation isperformed using a well known bias condition. In S215 step, an erroroccurs during a read operation and if an error occurred can not becorrected, S220 step is performed. In contrast, if an error does notoccur or an error occurred can be corrected, a read operation is over.

In S220 step, a read voltage (Vread′) applied to an upper word line anda lower word line of a select word line is changed. For example, when athreshold voltage of a nonvolatile memory cell decreases, a read voltage(Vread′) is decreased and when a threshold voltage of a nonvolatilememory cell increases, a read voltage (Vread′) is increased. A changedread voltage (Vread′) is stored in a nonvolatile memory controller, avolatile memory controller and a flash memory controller and can be usedduring a next read operation. In S225 step, a read operation isperformed by applying a changed read voltage (Vread′) to an upper wordline and a lower word line. In S230 step, an error occurs during a readoperation and if an error occurred can not be corrected, S235 step isperformed again. In contrast, if an error does not occur or an erroroccurred can be corrected, a read operation is over.

In S235 step, whether the number of changing a read voltage exceeds thereference count or not is detected. If the number of changing a readvoltage does not exceed the reference count, S220 step is performedagain. If the number of changing a read voltage exceeds the referencecount, S240 step is performed. In S240 step, a read reference voltage ischanged. For example, when a threshold voltage of a nonvolatile memorycell decreases, a read reference voltage (Vread′) is decreased and whena threshold voltage of a nonvolatile memory cell increases, a readreference voltage (Vread′) is increased. A changed reference readvoltage (Vread′) is stored in a nonvolatile memory controller, avolatile memory controller and a flash memory controller and can be usedduring a next read operation. In S245 step, a read operation isperformed again by applying a changed read reference voltage (Vread′) toa select word line. In S250 step, an error occurs during a readoperation and if an error occurred can not be corrected, S240 step isperformed again. In contrast, if an error does not occur or an erroroccurred can be corrected, a read operation is over.

In an embodiment according to the present inventive concept, a level ofa read voltage (Vread′) is changed according to whether an erroroccurred during a read operation can be corrected or not. A reduction ofa memory cell or an increased threshold voltage can be compensated bychanging a read voltage (Vread′). Also, if the number of changing a readvoltage (Vread′) exceeds the reference count, data stored in a memorycell can be accurately readout by changing a read reference voltage.

FIG. 21 is a flow chart illustrating a read method according to a tenthembodiment of a nonvolatile memory device according to the presentinventive concept. Referring to FIG. 21, a read method of a nonvolatilememory device according to the present inventive concept includes a stepof a read operation performance, an error detection step, a step of achanging a read voltage and a step of changing a read reference voltage.

In S310 step, a nonvolatile memory device receives a read instructionfrom external device. In S320 step, a nonvolatile memory device performsa read operation in response to a read instruction. A read operation isperformed using a well known bias condition. In S330 step, an erroroccurs during a read operation and if an error occurred can not becorrected, S340 step is performed. In contrast, if an error does notoccur or an error occurred can be corrected, a read operation is over.

In S340 step, a read voltage (Vread′) applied to an upper word line anda lower word line of a select word line is changed. For example, when athreshold voltage of a nonvolatile memory cell decreases, a read voltage(Vread′) is decreased and when a threshold voltage of a nonvolatilememory cell increases, a read voltage (Vread′) is increased. A changedread voltage (Vread′) is stored in a nonvolatile memory controller, avolatile memory controller and a flash memory controller and can be usedduring a next read operation.

In S350 step, a read reference voltage applied to a select word line ischanged. For example, when a threshold voltage of a nonvolatile memorycell decreases, a read voltage (Vread′) is decreased and when athreshold voltage of a nonvolatile memory cell increases, a read voltage(Vread′) is increased. A changed read reference voltage (Vread′) isstored in a nonvolatile memory controller, a volatile memory controllerand a flash memory controller and can be used during a next readoperation.

In S360 step, a read operation is performed using a changed read voltage(Vread′) and a read reference voltage. In S370 step, an error occursduring a read operation and if an error occurred can not be corrected,S340 step is performed again. In contrast, if an error does not occur oran error occurred can be corrected, a read operation is over.

In an embodiment according to the present inventive concept, if an erroroccurred during a read operation can not be corrected, a level of a readvoltage (Vread′) and a level of a read reference voltage are changedtogether. A reduction of a memory cell or an increased threshold voltagecan be compensated by changing a read voltage (Vread′). Also, datastored in a memory cell can be accurately readout by changing a readreference voltage.

FIG. 22 is a flow chart illustrating a read method according to aeleventh embodiment of a nonvolatile memory device according to thepresent inventive concept. Referring to FIG. 22, a read method of anonvolatile memory device according to the present inventive conceptincludes a step of a read operation performance, an error detectionstep, a step of a changing a read voltage and a step of changing a readreference voltage.

In 4310 step, a nonvolatile memory device receives a read instructionfrom external device. In S420 step, a nonvolatile memory device performsa read operation in response to a read instruction. A read operation isperformed using a well known bias condition. In S430 step, an erroroccurs during a read operation and if an error occurred can not becorrected, S440 step is performed. In contrast, if an error does notoccur or an error occurred can be corrected, a read operation is over.

In S440 step, a read voltage (Vread′) applied to an upper word line anda lower word line of a select word line is changed. For example, when athreshold voltage of a nonvolatile memory cell decreases, a read voltage(Vread′) is decreased and when a threshold voltage of a nonvolatilememory cell increases, a read voltage (Vread′) is increased. A changedread voltage (Vread′) is stored in a nonvolatile memory controller, avolatile memory controller and a flash memory controller and can be usedduring a next read operation.

In S450 step, a read operation is performed using a changed read voltage(Vread′). In S460, an error occurs during a read operation and if anerror occurred can not be corrected, S470 step is performed. Incontrast, if an error does not occur or an error occurred can becorrected, a read operation is over.

In S470 step, a read reference voltage applied to a select word line ischanged. For example, when a threshold voltage of a nonvolatile memorycell decreases, a read voltage (Vread′) is decreased and when athreshold voltage of a nonvolatile memory cell increases, a read voltage(Vread′) is increased. A changed read reference voltage (Vread′) isstored in a nonvolatile memory controller, a volatile memory controllerand a flash memory controller and can be used during a next readoperation.

In S480 step, a read operation is performed using a changed read voltage(Vread′) and a read reference voltage. In S490 step, an error occursduring a read operation and if an error occurred can not be corrected,S440 step is performed again. In contrast, if an error does not occur oran error occurred can be corrected, a read operation is over. In thepresent embodiment, a read voltage (Vread′) may be changed after a readreference voltage is changed or a read reference voltage may be changedafter a read voltage (Vread′) is changed.

In an embodiment according to the present inventive concept, if an erroroccurred during a read operation can not be corrected, a level of a readvoltage (Vread′) is changed. A reduction of a memory cell or anincreased threshold voltage can be compensated by changing a readvoltage (Vread′). Also, when an error occurs even by a change of a readvoltage (Vread′), a read reference voltage is changed. Data stored in amemory cell can be accurately readout by changing a read referencevoltage.

FIG. 23 is a flow chart illustrating a read method according to atwelfth embodiment of a nonvolatile memory device according to thepresent inventive concept. In the present embodiment, a nonvolatilememory device further includes a monitoring cell. The monitoring cell isdisposed in the memory cell array and has the same structure as thememory cell. Thus, a characteristic of a memory cell can be estimated bydetecting the monitoring cell. An initial data stored in the monitoringcell is known.

Referring to FIG. 23, a read method of a nonvolatile memory deviceaccording to the present inventive concept includes a step of reading amonitoring cell, an error detection step and a step of a changing a readvoltage. In S510 step, a nonvolatile memory device receives a readinstruction from an external device. In S520 step, a nonvolatile memorydevice changes a read voltage (Vread′) applied to an upper word line anda lower word line of a select word line. In S530 step, a read operationis performed on the monitoring cell. In S540 step, data read from themonitoring cell and a reference data are compared with each other. Ifthe data read from the monitoring cell is different from the referencedata, the S520 step is performed again. If the data read from themonitoring cell is the same as the reference data, a read operation isperformed on a memory cell.

In an embodiment according to the present inventive concept, data storedin the monitoring cell is read using a predetermined read voltage(Vread′). If data stored in the monitoring cell is accurately read, itmeans that a read voltage (Vread′) is proper. Thus, a read operation isperformed on a memory cell using a read voltage (Vread′). In contrast,if data stored in the monitoring cell is not accurately read, it meansthat a read voltage (Vread′) is not proper. Thus, a read voltage(Vread′) is changed. A reduction of a memory cell or an increasedthreshold voltage can be compensated by changing a read voltage(Vread′).

FIG. 24 is a flow chart illustrating a read method according to athirteenth embodiment of a nonvolatile memory device according to thepresent inventive concept. In the present embodiment, a nonvolatilememory device further includes a monitoring cell. The monitoring cell isdisposed in the memory cell array and has the same structure as thememory cell. Thus, a characteristic of a memory cell can be estimated bydetecting the monitoring cell. An initial data stored in the monitoringcell is known.

Referring to FIG. 24, a read method of a nonvolatile memory deviceaccording to the present inventive concept includes a step of reading amonitoring cell, an error detection step, a step of a changing a readvoltage and a step of changing a read reference voltage. In S610 step, anonvolatile memory device receives a read instruction from an externaldevice. In S620 step, a nonvolatile memory device changes a read voltage(Vread′) applied to an upper word line and a lower word line of a selectword line. In S630 step, a read operation is performed on the monitoringcell. In S640 step, data read from the monitoring cell and a referencedata are compared with each other. If the data read from the monitoringcell is different from the reference data, the S520 step is performedagain. If the data read from the monitoring cell is the same as thereference data, a read operation is performed on a memory cell.

In S650 step, whether the number of changing a read voltage exceeds thereference count or not is detected. If the number of changing a readvoltage does not exceed the reference count, S620 step is performedagain. If the number of changing a read voltage exceeds the referencecount, S660 step is performed. In S660 step, a read reference voltage ischanged and the S620 step is performed again.

In an embodiment according to the present inventive concept, data storedin the monitoring cell is read using a predetermined read voltage(Vread′). If data stored in the monitoring cell is accurately read, aread operation is performed on a memory cell using the predeterminedread voltage (Vread′). In contrast, if data stored in the monitoringcell is not accurately read, a read voltage (Vread′) and a readreference voltage are changed. A reduction of a memory cell or anincreased threshold voltage can be compensated by changing a readvoltage (Vread′) and a read reference voltage.

FIG. 25 is a block diagram illustrating a computing system including aflash memory according to the present inventive concept. Referring toFIG. 25, a computing system 300 includes a processor 310, a memorycontroller 320, input devices 330, output devices 340, a flash memory350 and a main memory device 360. In the drawing, a solid linerepresents a system bus through which data or an instruction passes.

The memory controller 320 and the flash memory 350 can constitute amemory card. The processor 310, the input devices 330, the outputdevices 340 and the main memory device 360 can constitute a host using amemory card as a memory device. The computing system 300 according tothe present inventive concept receives data from an external devicethrough input devices (e.g., a keyboard, a camera and so on). Inputteddata may be a multimedia data such as an instruction by a user or animage data by a camera. The inputted data is stored in the flash memory350 or the main memory device 360.

A result processed by the processor 310 is stored in the flash memory350 and the main memory device 360. The output devices 340 output datastored in the flash memory 350 and the main memory device 360. Theoutput devices 340 output digital data as a type that a man can sense.For example, the output device 340 includes a display or a speaker. Theread method according to the present inventive concept is applied to theflash memory 350. As reliability of the flash memory 350 is improved, anintegration and reliability of the computing system 300 is alsoimproved.

The flash memory 350 and/or the memory controller 320 may be mountedusing various types of packages such as PoP (package on package), ballgrid array (BGA), chip scale package (CSP), plastic leaded chip carrier(PLCC), plastic dual in-line package (PDIP), die in waffle pack, die inwafer form, chip on board (COB), ceramic dual in-line package (CERDIP),plastic metric quad flat pack (MQFP), thin quad flatpack (TQFP), smalloutline (SOIC), shrink small outline package (SSOP), thin small outline(TSOP), thin quad flat pack (TQFP), system in package (SIP), multi chippackage (MCP), wafer-level fabricated package (WFP), wafer-levelprocessed stack package (WSP).

A power supply portion (not shown) is also required to supply a powersupply which is necessary to an operation of the computing system 300.If the computing system 300 is a mobile device, a battery for supplyingan operation power supply of the computing system 300 is additionallyrequired.

FIG. 26 is a block diagram illustrating a structure of SSD systemaccording to the present inventive concept. Referring to FIG. 26, a SSDsystem 400 includes a SSD controller 410 and flash memories 420 through423.

A nonvolatile memory device according to the present inventive conceptmay be applied to a solid state drive (SSD). Recently, a SSD deviceanticipated to replace a hard disk drive (HDD) is in the limelight in anext generation market. A SSD stores data using memory chips such as aflash memory instead of a rotating plate used in a general hard diskdrive. A SSD has advantages of a high speed, low power consumption and astrong resistance against an external impact compared with a hard diskdrive (HDD) which is mechanically moving.

Referring back to FIG. 26, a central processing unit 411 determines orcontrols whether receiving data from a host to store the data from thehost in a flash memory or reading data stored in a flash memory totransmit it to the host. An ATA interface 412 exchanges data with thehost according to a control of the central processing unit 411. The ATAinterface 412 includes a serial ATA (S-ATA) standard and a parallel ATA(P-ATA) standard. The ATA interface 412 fetches an instruction and anaddress to transmit them to the central processing unit 411 through aCPU bus. Data inputted from a host through the ATA interface 412 or datato be transmitted to the host is transmitted through SRAM cache 413without passing through a CPU bus according to a control of the centralprocessing unit 411.

The SRAM cache 413 temporally stores data moving between the host andthe flash memories 420 through 423. Also, the SRAM cache 413 is used tostore a program to be put in practice by the central processing unit411. The SRAM cache 413 may be considered as a kind of a buffer memoryand is not always necessary to be comprised of SRAM. A flash interface414 exchanges data with nonvolatile memories used as a storage device.The flash interface 414 may be constituted so as to support a NAND flashmemory, one-NAND flash memory or a multi-level flash memory. Anonvolatile memory device according to the present inventive concept maybe used as a mobile storage device. Accordingly, the nonvolatile memorydevice according to the present inventive concept may be used as astorage device such as a MP3, a digital camera, a PDA, an e-Book, adigital TV or computer.

Having described the exemplary embodiments of the present invention, itis further noted that it is readily apparent to those of reasonableskill in the art that various modifications may be made withoutdeparting from the spirit and scope of the invention which is defined bythe metes and bounds of the appended claims.

1. A nonvolatile memory device comprising: a memory cell array connectedto a plurality of word lines; and a voltage generator for supplying aselect read voltage to a select word line and an unselect read voltageto an unselect word line among the plurality of word lines when a readoperation is performed, wherein the voltage generator generates theunselect read voltage having different level according to whether theunselect word line is adjacent to the select word line or not.
 2. Thenonvolatile memory device of claim 1, wherein the voltage generatorsupplies an unselect read voltage to at least one between an upper wordline and a lower word line adjacent to the select word line when athreshold voltage of a select memory cell connected to the select wordline is decreased, the unselect read voltage being lower than unselectread voltages supplied to other unselect word lines.
 3. The nonvolatilememory device of claim 1, wherein the voltage generator supplies anunselect read voltage to at least one between an upper word line and alower word line adjacent to the select word line when a thresholdvoltage of a select memory cell connected to the select word line isincreased, the unselect read voltage being higher than unselect readvoltages supplied to other unselect word lines.
 4. A read method of anonvolatile memory device, comprising: performing a first read operationby supplying a select read voltage to a select word line and supplyingan unselect read voltage to an unselect word line; detecting whether anerror occurs in the first read operation or not; and performing a secondread operation by changing a voltage of a word line adjacent to theselect word line when an error occurs in the first read operation. 5.The read method of claim 4, wherein the voltage generator supplies anunselect read voltage to at least one between an upper word line and alower word line adjacent to the select word line when a thresholdvoltage of a select memory cell connected to the select word line isdecreased, the unselect read voltage being lower than unselect readvoltages supplied to other unselect word lines.
 6. The read method ofclaim 4, wherein the voltage generator supplies an unselect read voltageto at least one between an upper word line and a lower word lineadjacent to the select word line when a threshold voltage of a selectmemory cell connected to the select word line is increased, the unselectread voltage being higher than unselect read voltages supplied to otherunselect word lines.
 7. The read method of claim 4, wherein whether anerror occurs in the first read operation or not is detected by an errorcorrection circuit.
 8. The read method of claim 4, further comprisingperforming the second read operation by changing the select read voltagesupplied to the select word line when an error occurs in the first readoperation.
 9. The read method of claim 4, further comprising performinga third read operation by changing the select read voltage supplied tothe select word line when an error occurs in the second read operation.10. The read method of claim 4, further comprising: detecting whetherthe number of changing a voltage of a word line adjacent to the selectword line exceeds the reference count or not when an error occurs in thesecond read operation; and performing a third read operation by changingthe select read voltage supplied to the select word line when the numberof changing a voltage of a word line adjacent to the select word lineexceeds the reference count.
 11. A read method of a nonvolatile memorydevice, comprising: performing a read operation by supplying a selectread voltage to a monitoring word line and supplying an unselect readvoltage to an unselect word line; detecting whether data stored in amonitoring cell connected to the monitoring word line is normallyreadout or not; and performing a read operation on the monitoring cellagain by changing a voltage of a word line adjacent to the monitoringword line when data stored in the monitoring cell is not normallyreadout.
 12. The read method of claim 11, further comprising performinga read operation on a select memory cell by supplying a voltage to aword line adjacent to a select word line when data stored in themonitoring cell is normally readout, the voltage having the same levelas the voltage of the word line adjacent to the monitoring word line.13. The read method of claim 11, further comprising performing a readoperation on the monitoring cell by changing the select read voltagesupplied to the monitoring word line when data stored in the monitoringcell is not normally readout and the number of changing the voltage ofthe word line adjacent to the monitoring word line exceeds the referencecount.